Transistor circuits utilizing shunt-input tunnel diode to provide positive switchingand improve rise time



Oct. 13, 1964 Filed Nov. 23, 1960 Rv F TRANSISTOR cIRcuITs U ARNswoRTH PROVIDE p0 3,153,151I LIZING SHUNT-INPUT TUNNEL DIODE T0 SITIVE SWITCHING AND IMPROVE RISE TIME 2 Sheets-Sheet 1 Oct. 13, 1964 R. P. FARNswoR-rl-l 3,153,151

TRANSISTOR CIRCUITS UTILIZING SHUNT-INPUT TUNNEL DIODE Y PROVIDE POSITIVE SWITCHING AND IMPROVE RISE TIME 'Filed Nov. 23, 1960 2 Sheets-Sheet 2 United States Patent O Y 3,153,151 'IRANSIS'IR CHICUITS UTILIZING SHUNTJN. PUT TUNNEL DIODE T PRVIDE POSITIVE SWI'ICHING AND ROVE RISE TIME Robert P. Farnsworth, Los Angeles, Calif., assignor to Hughes Air-craft Company, Culver City, Calif., a corporation of Delaware Filed Nov. 23, 1960, Ser. No. 71,256 2 Claims. (Cl. 307-885) This invention relates to transistor switching circuits and particularly to transistor circuits utilizing tunnel diode biasing arrangements for providing improved switching operation.

Conventionally a power transistor utilized for a switching operation has a biasing resistor coupled to the base through which leakage current Ico flows to a source of reference potential. In order to provide a circuit in which the transistor has a stable nonconductive condition, the biasing resistor is required to be relatively small so as to prevent the steady state leakage current from causing base injection into the transistor. However, while obtaining the stability advantages of a biasing resistor having a small value, a substantial amount of power is sacrificed from the driving source when the transistor is biased to a conductive state. Another disadvantage of the conventional transistor switching circuit is that the speed at which the output pulses rise .and fall is limited by the rise and fall times of the driving pulses.

It is therefore an object of this invention to provide an improved arrangement for rapidly switching a transistor.`

It is a further objectof this invention to combine a tunnel diode and `a transistor in a circuit so as to provide improved circuit characteristics during switching operation.

It is a still further `object of this invention to provide a tunnel diode biasing circuit for a switching transistor so as to develop reliable performance characteristics while utilizing a minimum power from the source of switching signals.

It is another object of this invention to provide a simplified and reliable switching circuit having characteristics of a conventional Schmitt trigger circuit.

The circuit in accordance with this invention includes a switching transistor combined with a biasing arrangement to provide a trigger type switching circuit. The biasing arrangement which functions to remove the reverse direction leakage current from the base of the transistor, includes a negative resistance device such as a tunnel diode coupled between the base and a source of reference potential. The tunnel diode controls the circuit characteristics so as to provide a proper source impedance both when the transistor is conductive or nonconductive. Thus, a low impedance is effectively presented to the leakageV current during the off condition so as to provide positive switching, and a high impedance is eifectively presented to the'switching source during the on condition so that the majority of the current from the source passes into the base of the transistor. Also, because of the negative resistance characteristics of the tunnel diode, the output pulses have fast rise and fall times. When the base' of the transistor is controlled by a voltage of varying amplitude rather than switching pulses, the circuits in accordance with this invention are a simplified and improved arrangement providing Schmitt trigger performance characteristics.

The novel features of this invention, as well as the invention itself, both as to its organization and method of operation, will best be understood from the accompany- 3,153,151 Patented Oct. 13, 1964 ing description taken in connection with the accompanying drawings,vin which:

FIG. 1 is a schematic circuit diagram of one arrangement of the transistor switching circuit in accordance with this invention;

FIG. 2 is a schematic circuit diagram of another arrangement of the switching circuit in accordance with this invention providing a large range of transistor leakage current than in the circuit of FIG. 1;

FIG. 3 is a schematic circuit diagram of another arrangement of the switching circuit in accordance with this invention, also providing a larger range of allowable transistor leakage current to the transistor than in the circuit of FIG. 1;

FIG. 4 is a graph of current versus voltage for explaining the operating characteristics of the circuits of FIGS. 1 and 3;

FIG. 5 is a graph of current versus voltage for explaining the operating characteristics of the circuit of FIG. 2;

FIG. 6 is a graph of waveforms for explaining the Schmitt trigger operating characteristics of the circuit of FIGS. l and 2;v and FIG. 7 is a graph of input voltage versus output voltage for further explaining the Schmitt trigger operation of the circuits of FIGS. 1 and 2.

Referring -iirst to the circuit of FIG. 1, an input signal source 12 is coupled through a resistor 14 and a lead 16 to the base of a transistor 20 which may be of the n-p-n type. The resistor 14 may be provided so that the source 12 is essentially a current source. The emitter of the transistor 20 is coupled to ground and the collector is coupled through a signal forming or load element such as resistor 24 toa terminal 26 which, for example, may supply a positive potential of -il0 volts relative to ground. The collector of the transistor 20 is also coupled through an output lead 28 to utilization circuits (not shown) to which pulses indicated by negative pulses of a waveform 30 are applied in response to positive input pulses indicated by a waveform 32. To 'provide substantially constant current biasing to the base of the transistor 20,

and to provide improved circuit characteristics, a negative resistance device such as a tunnel diode 34 has an anode coupled to the lead 16 and a cathode coupled to ground.

The tunnel diode 34 effectively changes impedance in response to the pulses of the waveform 32 so as to pass the desired transistor leakage current when the transistor Referring now to the circuit of FIG. 2 which allowsv operation with relatively larger transistor leakage currents than in the circuit of FIG. 1, an input signal source 38 is coupled through a resistor 40 to provided an effective current source and in turn through a lead 42 to the base of a transistor 44 which may be of the n-p-n type. The emitter of the transistor 44 is coupled to ground and the collector is coupled through a signal forming or load element such as a resistor 46 to a terminal 48 which may provide a positive potential of +10 volts. The collector of the transistor 44 is also coupled through output lead 50 to utilization circuits (not shown) so as to apply negative output or load pulses indicated by a waveform 52 thereto in response to positive input pulses on the lead 42 indicated by a waveform 54. In order to provide improved operating characteristics to the transistor 44, a tunnel diode 56 is provided with an anode coupled to the lead 42 and a cathode coupled to ground. To further improve the circuit operating characteristics so as to tolerate increased transistor leakage current, a resistor 58 is coupled between the lead 42 and ground.

A further arrangement in accordance with this inven- E tion'shown inFIG. 3 and operable with a relatively large leakage current from the transistor, includes both an input pulse source 60 and a trigger pulse source62 coupled to end of the coil 78 is coupled through a lead 82 to the base y of a transistor 84 which may be of the n-p-n type. The trigger signals developed by the source 62 are applied through a lead 86 to one end' of a coil 8S of the transformer St) and therethrough -to ground. The transformer 8i) yhas a polarity relation indicatedby dats-92 and 94 such that lthe negative trigger signal of the waveform 72 is combined with the pulses of the Waveform 76 to form pulses on theflead 32 yindicated by a waveforrn having a negative spike at the trailing-edge of each pulse. As will be discussed subsequently, this negative spike causes the source 60 to draw momentary reverse current so that the transistor 8'4'switches tothe nonconductive condition in the presence of relatively large leakage current requirements. The collectorfof the transistor 84 is coupled to a terminal which may be at a potential'of +10 volts, through a signal forming or load element such as a resistor 1112. The emitter of the transistor v84 is referenced 4to ground. Also, the collector ofthe transistor $4 is coupled through an output 4lead1tl6 to utilizationcircuits (not shown) to which is vapplied negative outputpulses indicated by-al wavetorrn'lti.

For further explaining the switching operation ofthe circuit of FIG. 1 in laccordance with this invention, the graph of FIG. 4 includes va curve 114 showing vthe operating characteristics of the tunnel diode34 and a curve 116 showing the operating characteristic of the base current Ibc 'versus base-to-emitter voltage Vb@ of the transistor 20. Another curve 118 "drawn to the same Vscale but with a brokenpportion at higher currentsishowsthe collector current Ic versus base-to-emitter voltage Vhs, which indicates the load current owing'- through the resistor 24. Because of the arrangement of the tunnel diode 34 and the transistor Ztl in accordancewith this invention, the circuit operating characteristics assume that indicated by a composite curve 12) which is a combination of the tunnel diode characteristic curve 114 and the base current versus base-to-emitter kvoltage of the transistor 20. The curve 120 represents parallel performanceV of the base-to-emitter of the transistor and the tunnel diode 34 relative to the lead 16. It is to be noted that :a valley point 134 `of the curve 120 is substantially to the left of the valley point of the curve 114.

When the transistor 20 is biased in a nonconductive state Vby the lower potential'level shown by the waveform 32, the tunnel diode 34 Vis maintained at a point such as 122 as a result of transistor leakage current which for the arrangement of FIG. l is necessarily at a current level below the valley point of the curve 126. The point 122 is on the composite curve 120 Which in the region thereof -760 is essentially the sameas the tunnel diode curve 114 because the base-to-emitter current of the Vtransistor'20 shown by curve 116 is Zero. Upon application of a positive pulse of the waveform 32, the voltage developed across the tunnel diode increases to the peak 124 of the' curve 114, which is the point where the incremental rate of change of current relative to` voltage equals zero and becomes negative. As a result the operating point of the circuit changes state to a point such as 126 on the curve 120. -In the conductive state represented by the point 126 onthe composite curve 120, the tunnel diode current indicated by an arrow 127 may be'approximately the same as the tunnel diode current at point 122 and the base-toemitter current of the transistor 2t) indicated by an` arrow Y 128 is the majority ofthe current-from the source 12.-

the tunnel diode 34 eectively develops a high impedance so that only a small part of the current that would pass through a conventional biasing resistor from the source 12 is passed through the tunnel diode 34. While the circuit is operating at the point '126, the collector current of the transistor 2t) is at a point 130 along a saturation line 132 and current passing through the resistor 24 develops an lamplified negative pulse of the Waveform 30. It is to be noted that if the peak ,ofl the pulse of the waveform 32 varies, the point 126 moves along the composite curve 1211 with a relativelyA small increase of current through the tunnel diode 34 while the collector current moving along-the saturation line 132 remains substantially constant. Y

At the termination of' the pulse of the waveform 32 the point 126 falls along the composite curve 120 to a valley point 134 at which the sum of the incremental rates of change of current with respect to voltagel resulting from the nonlinear impedance of the tunnel diode as shown by curve `114 and the nonlinearimpedance of the curve 116 equals zero and becomes negative, causing the tunnel diode 34 to change state andthe circuit operating point to be triggered tothe first point 122. The incremental change of current onV the lead 16 is equal to the incremental change of base-to-emitter current of the transistor 21D plus the incremental change of current through the tunnel diode 34. The incremental change in current Al of the input is equal to dl/ dV of the transistor 20 times the voltage change at the lead 16 plus dI/dV of the tunnel diode 34 times the voltage change at lead 16. The circuit remains stable in the conductive state as long as the incremental rate of change of current in the input lead 16 is negative with negative incremental voltage such as between points'126 and 134 of the composite curve 12). During switchingfrom pointsv126 to'134 the transistor base-to-emitter incremental current change is negative and the tunnel diode incremental current change is positive. As the operating point of the circuit falls to valley point 134, the vincremental change of tunnel'diode current with decreasing voltage becomes equal to the incremental change oftransistor base current with decreasing voltage n and the incrementalchange of input current becomes positive, thusycausing the tunnel diode 34 to switch to point 122.

Therefore, because the circuit of FIG. 1 only switches to an oi condition when the'net rate of current change with decreasing voltage is negative, the point 122 representing the reverse direction leakage current Icoowing from the base the transistor 20 must be maintained below the valley point 134. In the switching operation of the circuit of FIG. 1 the combined characteristics of the tunnel diode 34 and the transistor 20 provides an effective low impedance at the point 122 for the transistor leakvage current Ico when the transistor 20 is nonconductive and provides an e'ie'ctive high impedance to current from the source 12 when the transistor 20 isswitched into conduction such as at the point 126,-so as to effect a substantial saving of power from the source 12. Also, because of the trigger action of the circuit and because the collector current remains substantially constant along the saturation line 132, the pulses of the waveform 30 have steep leading and trailing edges without being limited by the steepness of the input pulses `of the waveform 32.

Because the leakage current in the circuit of FIG. 1 must be maintained below the valley point 134 in order for the transistor 20 to switch to the oit or nonconductive condition as discussed above, the arrangement of FIG. 2 is provided to allow the leakage current to be substantially larger such `as in high temperature operation. The graph of FIG. 5 shows a curve 138 representing the inherent operating Vcharacteristics of the tunnel diode 56, a curve indicating the base current I),e versus the base-toemitter voltage Vbe ofthe transistor 44 and a curve 142 indicating the linear current variation through the parallel resistor 58. Because of the circuit arrangement of FIG. 2, the operating characteristic thereof changes so that the circuit characteristic is indicated by a composite curve 144 representing parallel operation relative to the lead'42. The curve 144 which is a composite ofthe curves 142, 13S and 140 has a valley point 148, which is at a substantially higher current than the valley point of a curve 149 which is a composite of only the tunnel diode current and base-to-ernitter current. Thus, in switching operation large leakage current may be tolerated up to the level at the point 148 while allowing switching of the circuit from the conductive to nonconductive state. When the transistor 44 is maintained nonconductive, the tunnel diode may be at a characteristic operating point 150 on the composite curve 144 allowing a relatively large leakage current ICO to pass therethrough, and upon application of a positive pulse of the waveform 54 the circuit changes state rapidly to a point 152. At the saine time the collector current of the transistor 44 when biased into conduction operates on a saturation line 154 at a point 156 to form the ,constant current negative output pulse of the waveform 52. Y

At the termination of the pulse of the waveform 54 the operating point of the circuit falls along the curve 144 to the level of the valley current of the point 148, at which the rate of change of current with respect to voltage is zero, and then returns rapidly to the operating point 15). As a result, the transistor 44 is rapidly switched out of conduction to terminate the output pulse of the waveform 52. Because of the rapid change of state of the circuit in accordance with this invention, the inverted output pulses of the waveform 52 have rapid tallV and rise times that may be substantially more rapid than that of the input pulse of the waveform 32. It is to be noted that at the operating point such as 152 the current through the tunnel diode is relatively small but that a substantial amount of current passes through the resistor 5S. A relatively large current passes from the lead 42 into the base of the transistor 44. Also at the operating point 150 a small amount of current passes through the resistor 58 partly `from the source 3S. Thus, by sacricing current from the source 38 through the resistor 58 much larger transistor leakage currents may be tolerated. However, the combined current through the tunnel diode and through the resistor 5S when the transistor 44 is switched to the conductive condition is substantially less than when utilizing a lixed bias resistor in the conventional arrangement.

In another arrangement for allowing a relatively large leakage current to be tolerated from the switching traneA Y tunnel diode 83 tochange state and the transistor 84 to be shifted to the off or nonconductive condition.

The principles in accordance with this invention having` been explained relative to their performancevin response to switching pulses, the operation will now be explained in response to aY varying input potential to develop the characteristics of a Schmitt trigger circuit, that is, a circuit that triggers in one direction when the input poten? tial is raised to a critical value and triggers in the reverse direction when the input potential is reduced to another lower critical value. First considering the circuit of FIG. 1, an input potential of a waveform 166 as shown in FIG. 6 is applied to the resistor 14 at the source 12 when the circuit is at the operating point 122 of FIG. 4, providing a potential rise on the lead` 16 as shown by a waveform 167. When Vthe potential applied from the source 12 rises to an upper trigger point 168, the tunnel i ever, as discussed above relative to FIG. 5, the circuit sistor as shown in FIG. 3, negative trigger pulses of the Y waveform 72 are combined at the trailing edges of the pulses of the waveform 76 to form the combined pulses of the waveform 9S. This negative trigger pulse or spike causes the source titl to receive a momentary reverse current so that the circuit will change from the conductive to the nonconductive condition. As shown in FIG. 4, a high leakage current may provide operation at a point 152, for example, substantially above the valley point 134 of the composite curve 120. Similar to the discussion relative to FIG. 1 when the pulse 9S is applied to the lead 82, the operating point changes rapidly to the point 126 during formation of the output pulse of the waveform 119. At the termination of the pulse of the waveform 9S the negative potential applied to the anode of the tunnel diode 33 provides a momentary negative incremental current change to cause the operating characteristic of the circuit to shift down to the left on the curve 121i and then rapidly to the oi point 162. It is again to be noted that the pulses of the waveform 110 have fast rise times and fall times because of the rapid change of the circuit from one state to another. Thus, in the arrangement of FIG. 3, the leakage current may be substantially larger than the current level at the valley point 134 with the negative trigger pulse allowing the diode 34 changes state to the operating point 126 of PIG. 4 and the transistor 20 conducts load current to develop a negative output pulse of waveform on the lead 28. Regardless of the rise in'potential of the input potential of thenwaveform 166, as shown by a dotted line, the transistor Ztl remains saturated. As the potential of the waveform 166 drops to a lower trigger point 174, the tunnel diode 34 changes state to the point 122. Also, when the circuit is operating at the point 122, regardless to what lower level that the potential of waveform 166 falls, as indicated bya dotted line, the transistor 25B remains biased in the nonconductive state.

To further illustrate the operation of the circuit of FIG. 1 lfor developing a Schmitt trigger characteristic, a curve 178 of FIG. 7 shows the two output levels of the -pulse i176 in response to the varying potential levels of the waveform 166. When the transistor 20 is nonconductive, the out-putpotential level V0 is high as indicated at 179 and when the transistor 20 is conductive, the output pulse of the waveform 170 indicated at the level 180 is developed. Thus, when the input potential V1 rises to the upper trigger point i168 (PIG. 6) the curve 178 falls to the -low voltage level 180 and regardless of variations of input potential remains at that output level until the input potential vfalls to the lower trigger point 174 (FIG. 6). The circuit then switches to the high output voltage level 179 in which condition the transistor 20 is in the off or nonconductive condition.

The circuit of FIG. 2 providing operation v'with a relaltively large transistor leakage current also has similar Schmitt characteristics as illustrated by FIG. 7. How

of FIG. 2 switches [from the low voltage state to the high voltage state while allowing a relatively large transistor leakage current to liow through the tunnel diode 56. It is to be noted that the hysteresis of the circuit which is the voltage between the lower and upper trigger points may be selected in the circuit of FIG. 2 by varying the value of the parallel resistor 58 so that the valley point 148 of -FIG. 5 is at a desired current.

Although the circuits in accordance with this invention have been explained when utilizing n-p-n type transistors, it is to be noted thatopposite polarity type iransistors may be utilized within the principles of this invention by reversing the polarity relations of the circuit.

Thus, there has been described an improved transistor and tunnel diode circuit `,that develops controlled current biasing for the transistor. Substantial power is saved from the driving source and the switching transistor has effective zero collector current in the olf or nonconductive condition. Because the valley point of tunnel diodes has been yfound to be relatively low, the circuits in accordance with this invention allowing a relatively large leakage current to be tolerated through the transistor will be highly useful. Also, the circuits in accordance with this invention provide a trigger type operation so as to develop output pulses with steep leading and trailing edges independent of the steepness ot the input pulse,

and to develop with Schmitt trigger characteristics.

What is claimed is:

1. A circuit for developing output pulses in response to input pulses comprising a first input'source for developing input pulses, a second input source couple-d to said first source for developing trigger pulses at the termination of lcachot the input pulses developed by said irst source, a source or" potential having first and second terminals, a transistor having a base, an emitter and a collector with the base coupled to one terminal of said first input source and the emittercoupled to one end of said lirst terminal of said source of potential, limpedance means coupled between said second terminal Vof said source of potential and the collector of said transistor, a tunnel diode coupled between the base of said transistor and said irst terminal of saidsource of potential, and

, iirst and second mutually coupled inductors with said first inductor coupled between said one terminal of said rst input source and the base of said transistor and the sec,

ond inductor coupled between one terminal of said second source of input signals and said lirst terminal of said source of potential, said first and second input sources land said rst and second mutually coupled inductors bef ing .intercoupled so that said input pulse and said trigger pulse are combined to provide a composite lwaveform having a first portion of one polarity and -a second portion of relatively short duration and of opposite polarity from said lirst por-tion immediately ttollowing said iirst portion.

2. A switching circuit responsive to input pulses to form `out-put pulses comprising a rstinput source fordeveloping input pulses, a second input source coupled to said iirst source ttor developing a trigger pulse of opposite polarity from said input pulses at the termination of each of said input pulses, a source of positive potential having -irst and second terminals, a transistor having a base, `an emitter and a collector with the emitter coupled to said `first terminal of said source of positive potential, a resistor coupled between -said second terminal of said source of positive potential and the collector of said transistor, a tunnel diode having an anode and a cathode coupled respectively lbetween the base of said transistor and said `tirst ter-minal of said source of positive potential, lirst and second inductive elements inductively coupled with said first inductive element coupled between one terminal of said first input source and the base of said transistor and with said second inductive element coupled between one terminal of said second inputsource and said rst terminal of said source lof positive potential, Isaid rst and sec- .ond inductive elements being intercoupled so `that said input pulse and said trigger pulse are combined to provide at the base of said transistor a composite wave-form having a tirst portion of one polarity and a second portion of relatively short duration and of` opposite polarity trom said first portion immediately following said iirst portion.

References lCited in the le of this patent UNITED STATES PATENTS v2,843,765 Aigrain July 15, s 2,956,175 Bouwen oct. 111, 1960 2,975,377 Price Mar. 14, 1961 OTHER. REFERENCES Hughes Tunnel Diode Application Engineering Notes L-9, May 1960.

Sylvan et al.: Tunnel Diodes as Ampliers and Switches. Reprint from May 1960, Issue of Electronic Equipment Engineering; 7 ppt-only pp. 6 and 7.

Neff et al.: Esaki Diode Logic Circuits, 1960, Int. Solid State Circuits Conf.; pages 16 and 17. 

1. A CIRCUIT FOR DEVELOPING OUTPUT PULSES IN RESPONSE TO INPUT PULSES COMPRISING A FIRST INPUT SOURCE FOR DEVELOPING INPUT PULSES, A SECOND INPUT SOURCE COUPLED TO SAID FIRST SOURCE FOR DEVELOPING TRIGGER PULSES AT THE TERMINATION OF EACH OF THE INPUT PULSES DEVELOPED BY SAID FIRST SOURCE, A SOURCE OF POTENTIAL HAVING FIRST AND SECOND TERMINALS, A TRANSISTOR HAVING A BASE, AN EMITTER AND A COLLECTOR WITH THE BASE COUPLED TO ONE TERMINAL OF SAID FIRST INPUT SOURCE AND THE EMITTER COUPLED TO ONE END OF SAID FIRST TERMINAL OF SAID SOURCE OF POTENTIAL, IMPEDANCE MEANS COUPLED BETWEEN SAID SECOND TERMINAL OF SAID SOURCE OF POTENTIAL AND THE COLLECTOR OF SAID TRANSISTOR, A TUNNEL DIODE COUPLED BETWEEN THE BASE OF SAID TRANSISTOR AND SAID FIRST TERMINAL OF SAID SOURCE OF POTENTIAL, AND FIRST AND SECOND MUTUALLY COUPLED INDUCTORS WITH SAID FIRST INDUCTOR COUPLED BETWEEN SAID ONE TERMINAL OF SAID FIRST INPUT SOURCE AND THE BASE OF SAID TRANSISTOR AND THE SECOND INDUCTOR COUPLED BETWEEN ONE TERMINAL OF SAID SECOND SOURCE OF INPUT SIGNALS AND SAID FIRST TERMINAL OF SAID SOURCE OF POTENTIAL, SAID FIRST AND SECOND INPUT SOURCES AND SAID FIRST AND SECOND MUTUALLY COUPLED INDUCTORS BEING INTERCOUPLED SO THAT SAID INPUT PULSE AND SAID TRIGGER PULSE ARE COMBINED TO PROVIDE A COMPOSITE WAVEFORM HAVING A FIRST PORTION OF ONE POLARITY AND A SECOND PORTION OF RELATIVELY SHORT DURATION AND OF OPPOSITE POLARITY FROM SAID FIRST PORTION IMMEDIATELY FOLLOWING SAID FIRST PORTION. 